Synthesis And Application Of Novel Fluorescent Carbon Quantum Dots In Biosensoring And Celluar Imaging

As a new“zero-dimensional”carbon nanomaterial,carbon quantum dots have been widely used as fluorescent probes for detecting various analytes in the environment or biological systems due to their intrinsic fluorescent properties,low toxicity,good biocompatibility,high sensitivity,quick response,low cost and simple preparation methods.The small size,large specific surface area,and abundant surface functional groups make carbon quantum dots very reactive and sensitive to the surrounding environment such as temperature,ionic strength,and solvent,resulting in changes to their optical properties.The basic analytical principles of fluorescence sensors can be summarized as the interaction between recognition components and targets which induces the changes of the fluorescence properties of carbon quantum dots and the changes quantitatively relate to the concentration or structure of the target analytes.The four main fluorescence signal outputs are fluorescence quenching,fluorescence enhancement,the ratiometric fluorescence and the emission wavelength shift.In this work,we synthesized three different functionalized carbon quantum dots using a one-step method.On the basis of sufficient research on their fluorescence properties,we established turn on,turn-off and ratiometric types fluorescent sensor for the detection of trypsin,sialic acid and the imaging of lysosomal p H in living cells.The specific researches are as follows:We propose a fluorescence turn-on strategy for the sensitive and selective detection of trypsin based on Inner filter effect(Inner Filter Effect,IFE)between amino-functionalized carbon quantum dots and gold nanoparticles(Au NPs).In this system,carbon quantum dots were treated as fluorometric reporter,while Au NPs were treated as fluorescence quencher.A specially designed negatively charged short peptide chain with one functioned cysteine group as the substrate for trypsin.The fluorescent sensor works as follow:the negatively charged part of the peptide can be hydrolyzed by trypsin,resulting in the releasing of shorter and positively charged peptides containing cysteine group.These shorter peptides then induce Au NPs aggregation due to the strong electrostatic interaction and Au-S bond between the negatively charged Au NPs and positively charged peptides,thus leading to the restoration of the IFE-quenched fluorescence emission of carbon quantum dots.Measuring the changes in the fluorescence of carbon quantum dots,the concentrations of trypsin can be determined.Under the optimized conditions,we achieved quantitative evaluation of trypsin in a range of 2.5-80 ng m L^-1 with the detection limit of 0.84 ng m L^-1.Meanwhile,this sensing system also exhibited excellent selectivity and sensitivity for trypsin,and we successfully applied it for the detection of trypsin in spiked human serum samples.Based on the above findings,we conclude that this sensor might be potential method for diagnosis of trypsin-related disease in the future.Typically,sialic acids(SA)with nine-carbon backbone are found at the glycan chain termini on the cell membranes,which play crucial roles in various physiological and pathological processes.In addition,SA regarded as a tumor-associated biomarker as their overexpression in blood serum can reveal a malfunction of several organs of the human body and also indicate an early stage of various cancers or cardiovascular disease.In this study,a novel turn-off approach for preparing fluorescent boronic-acid-modified carbon quantum dots based on static quenching mechanism.The functionalized carbon quantum dots were synthesized by a facile,one-step hydrothermal method using 3-pyridineboronic acid as the sole precursor.The added SA selectively recognized the carbon quantum dots,leading to the fluorescence static quenching of the carbon quantum dots in a linear range of 80-3000?mol L^-1 with a detection limit of 26?mol L^-1.Moreover,the developed boronic-acid fluorescent sensor was successfully applied for the detection of SA in human serum samples,the recovery ranged from 92.5 to 102%.In addition,this method afforded results within 4 min.Compared to other colorimetric or fluorescent methods,this new proposed approach was simpler,rapider and exhibited excellent sensitivity and selectivity,demonstrating immense potential as an alternative for SA detection.A hydrothermal method has been employed to synthesize a one-pot carbon quantum dots-based ratiometric sensor for imaging and monitoring lysosomal p H in living cells.The carbon quantum dots were directly functionalized by abundant amino groups during synthesis and exhibited dual emission bands at 550 nm and 439 nm under single-wavelength excitation of 380 nm without any additional modification.As the most acidic organelles in the cells,the lysosomal p H is in the range of 4.4-5.5.Therefore,a p H probe containing amino groups could better target such acidic environment.Owing to its abundant amino groups and good biocompatibility,the sensor is able to target lysosomes with high Pearson's coefficients(0.935 and 0.924)and ratiometrically responds to the change of lysosomal p H in living cells.In the range of p H 2-6.6,the fluorescence intensity ratio of carbon quantum dots(F₅₅₀/F₄₃₉)and p H exhibited a good linear relationship.And this linear range is within the p H range of the lysosome.It also had excellent p H sensitivity and reversibility,and anti-interference capability,thus enabling sensing p H change in intracellular environment in real time,as demonstrated by successfully monitoring of lysosomal p H changes during lysosomal alkalization,dexamethasone-induced stimulation,and stress in Michigan Cancer Foundation-7.